Epoxy resin laminate sheets comprising as a substrate a nonwoven fabric of glass fibers have heretofore been prepared by a method comprising the steps of impregnating a nonwoven fabric of glass fiber with an epoxy resin, drying the impregnated glass fibers to form a prepreg, laminating a predetermined number of so formed prepregs, optionally piling a metal foil such as copper foil on the laminate, and then pressing, while heating, the laminate between mirror-polished iron plates. Laminate sheets prepared according to this conventional method with use of a nonwoven fabric of glass fiber have good heat resistance comparable to that of epoxy resin laminates prepared by employing a woven fabric of glass fibers as a substrate, and they are excellent in drilling workability and punching workability.
Epoxy resin laminate sheets prepared according to the above conventional method by employing a nonwoven fabric of glass fiber as a substrate, however, are very poor in moisture resistance, and they are fatally defective in that when they are subjected to the boiling water test, reduction of the insulation resistance is extreme and the substrate is whitened. Therefore, when these laminate sheets are used for formation of printed circuit boards, they hardly satisfy requirement standards specified with respect to epoxy resin laminate sheets comprising a substrate of a glass fiber woven fabric or a paper substrate for use in production of printed circuit boards.
It is believed that the above fatal defect of an epoxy resin laminate sheet comprising a nonwoven fabric of glass fiber as a substrate is due to a binder used for formation of the nonwoven fabric of glass fiber. The nonwoven fabric of glass fiber used in this field includes a glass mat and a glass paper, and it is generally formed by shaping glass fibers having a diameter of several microns or shortly cut products thereof into a thin sheet or paper. It is known that at this step, as a medium for bonding the glass fibers, there are employed (1) chemicals or resin (the product is hereinafter referred to as "substrate A") and (2) a cellulose fiber (the product is hereinafter referred to as "substrate B"). Such bonding medium of binder has bad influences on properties of laminate sheets for use in producing printed circuit boards. A method comprising treating a nonwoven fabric of glass fiber with a silane coupler may be considered as means for improving this defect. According to this method, however, in the case of the substrate A a large amount of the silane coupler should be used for the treatment and the manufacturing cost becomes high, and in the case of the substrate B no improvement of properties can be attained.
We have made investigations on epoxy resin laminate sheets comprising a nonwoven fabric of glass fiber as a substrate, expecially those for printed circuit boards, with a view to developing a method in which the foregoing fatal defect can be overcome and the properties of these epoxy resin laminate sheets can be improved at a lower cost, and as a result it has now been found that good results can be obtained when a nonwoven fabric of glass fiber to be used to be used as a substrate is treated with a liquid mixture containing a watersoluble phenol resin and a silance coupler.